1. Field of the Invention
This invention relates to protective layers applied to optical coatings on transparent substrates. In particular, the invention relates to the use of a temporary protective layer of carbon. In addition, the invention relates to a scratch propagation blocker (SPB) protective layer applied to the outermost layer of various optical coatings.
2. Discussion of the Background
Optical coatings are deposited on transparent substrates to reflect or otherwise alter the transmission of some or all of the radiation incident on the substrates. For example, the optical coating of a mirror is designed to reflect visible light. Low-emissivity optical coatings are designed to reduce the transmission of infrared radiation. Optical coatings generally include two or more different layers each having a thickness in a range of from less than 1 nm to over 500 nm.
Optical coatings are frequently damaged during shipping and handling by scratching and by exposure to corrosive environments. Silver based low-emissivity coatings in particular have been plagued with corrosion problems since their introduction into the fenestration marketplace decades ago. Attempts at improving the durability of optical coatings have included the application of a temporary protective layer such as a plastic adhesive backed film. Other protective layers have been formed by applying and curing solvent based polymers on glass.
However, a number of problems are associated with using adhesive films and polymer films as protective layers on optical coatings. Expensive, specialized equipment is required to apply the adhesive films and the polymer films to optical coatings. When an adhesive film is pulled away from an optical coating, the adhesive film runs the risk of removing portions of the optical coating. Even if portions of the optical coating are not removed, the force on the optical coating associated with removing the adhesive film can damage the optical coating. A solvent based polymer film applied to an optical coating must be dried and the solvent removed in an environmentally friendly manner. Removal of the polymer film from an optical coating requires specialized washing that can easily damage the optical coating.
For protection from corrosion, most silver based low-emissivity stacks in use today make use of barrier or cladding layers in direct contact and on one or both sides of the silver layers. It is well known in the art that various thin film layers can function as barriers to movement of corrosive fluids such as water vapor and oxygen. Metals layers are known to be particularly effective diffusion barriers due to their ability to physically and chemically inhibit diffusion of corrosive fluids. Metal layers tend to be more effective physical barriers to diffusion than dielectric layers such as oxides, because both evaporated and sputtered metal layers tend to contain fewer pinhole defects than oxide layers. Metal layers also tend to chemically block diffusion by reacting with fluids diffusing through a pinhole to stop the movement of all chemically bound fluid molecules. The bound fluid molecules in turn restrict the passage of additional fluid through the pinhole. The more reactive metals are particularly effective for chemically blocking.
Tempering greatly reduces the corrosion problems associated with silver based low-emissivity coatings. Tempering results in an atomic level restructuring to a lower energy state and renders the silver far less prone to corrosion. Tempering also improves the hardness and scratch resistance of optical coatings.
However, until optical coatings are tempered, the coatings remain particularly susceptible to damage from scratching and corrosion. Even after tempering, optical coatings are not immune from scratching and corrosion.
Scratches in an optical coating frequently do not become visible until after the coating is heated and tempered, which can cause the scratches to grow and propagate.
Carbon has been used as a protective coating on glass substrates. For example, U.S. Pat. No. 6,303,226 discloses the use of an amorphous, diamond-like carbon (DLC), protective layer on a glass substrate.
There is a need for improved methods and layers for protecting optical coatings.
The present invention provides a method of making a transparent article with a reduced number of scratches and other surface defects. The transparent article includes a optical coating on a transparent substrate. According to the invention, a protective coating is formed on the optical coating that improves the durability and scratch resistance of the optical coating, particularly during manufacturing.
The protective coating can include a layer consisting essentially of carbon. The carbon protective layer is formed on the optical coating before tempering. During shipping and handling of the untempered optical coating, the carbon layer serves as a low friction, protective layer against scratches. Heating and tempering the optical coating and/or transparent substrate in an atmosphere reactive to carbon consumes the carbon protective layer, thus eliminating any scratches or other surface defects in the carbon. The carbon protective layer is converted into a carbon containing gas, leaving behind a relatively scratch-free optical coating.
The protective coating can also include a thin protective layer of a scratch propagation blocker (SPB) material. The SPB material inhibits the propagation of scratches into the brittle, glassy, outermost layer of various optical coatings during tempering. SPB materials such as Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W, and oxides and nitrides thereof, are suitable for use on an outermost layer of silicon nitride (e.g., Si3N4). The SPB layer can be formed by depositing on the outermost layer of an optical coating a diffusion barrier layer of at least one metal, metal sub-oxide or metal sub-nitride of Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta or W; and then reacting the diffusion barrier layer with an oxygen containing atmosphere such as air to form a metal oxide SPB layer including at least one of TiO2, SiO2, ZnO, SnO2, In2O3, ZrO2, Al2O3, Cr2O3, Nb2O5, MoO3, HfO2, Ta2O5 and WO3. The SPB layer can be used with or without a carbon protective layer on the SPB layer.
Use of the temporary carbon protective layer when manufacturing a transparent article having an optical coating significantly reduces the number and severity of scratches introduced into the optical coating by the manufacturing process. Because the carbon layer is removed during tempering, the carbon layer does not affect the optical properties of the optical coating. While the SPB layer is not removed during tempering and may affect the optical properties of an optical coating, the SPB layer, by inhibiting scratch propagation, is particularly useful in protecting a brittle, glassy, outermost layer of an optical coating from the formation of visible scratches. A metal, metal sub-oxide or metal sub-nitride layer is particularly useful in providing corrosion protection before tempering and can be converted by tempering in an atmosphere containing oxygen to a metal oxide SPB layer that is essentially transparent to visible light.